The first Global System for Mobile (GSM) communication networks were designed for voice services rather than for data services. When the use of GSM data services started, it soon became evident that the Circuit Switched (CS) bearer services were not well-suited for certain types of applications with a bursty nature. Therefore the new Packet Switched (PS) data transmission service GPRS (General Packet Radio Service) was developed for packet services. GPRS is a packet radio network utilizing the GSM network, and GPRS endeavours to optimize data packet transmission by means of GPRS protocol layers on the air interface between a mobile station (hereinafter also called a mobile terminal) and a GPRS network.
A GPRS mobile station (MS), also called a mobile terminal, can operate in one of three modes of operation, as described in 3GPP TS 23.060, “Service description; Stage 2,” Section 5.4.5. The three modes are Class-A Mode, Class-B Mode, and Class-C Mode. According to the Class-A mode of operation, the MS is attached to both GPRS as well as other GSM services, and therefore Class-A Mode corresponds to Dual Transfer Mode (DTM) (hereinafter also called dual mode). The mobile user in Class-A Mode can make and/or receive calls on the two services simultaneously, for example having a normal GSM voice call and receiving GPRS data packets at the same time. According to the Class B mode of operation, the MS is attached to both GPRS and other GSM services, but the MS can only operate one set of services at a time. According to the Class C mode of operation, the MS can only be attached either to the GSM network or the GPRS network; the selection is done manually and there are no simultaneous operations.
Based on the current standard (3GPP TS 44.018, “Radio Resource Control Protocol”), when the MS releases a CS connection (also referred to as a radio resource or RR connection) while in the Dual Transfer Mode (DTM), all packet resources are aborted. This is illustrated in FIG. 1 (also see 3GPP TS 43.064, “Overall description of the GPRS radio interface; Stage 2”), which shows RR operating modes and transitions between Class-A (DTM supported) and Class-B. An RR Release moves the MS from the Dual Transfer Mode 102 into an Idle/Packet Idle state 104, after which the MS must then obtain packet access in order to perform packet transfer. In other words, after the release of the CS connection, the MS is in the packet idle mode and must perform a complete acquisition of system information and ask for PS resources again, in order to get into the Packet Transfer Mode 106.
FIG. 2 further illustrates how the system is currently working, according to the prior art. The four vertical lines represent portions or stages of the network. The line 202 represents the mobile station (MS), the line 204 represents the base station system (BSS), the line 206 represents the serving GPRS support node (SGSN), and the line 208 represents the mobile switching center (MSC). FIG. 2 shows that initially a packet switched session 210 and a circuit switched session 212 are in progress according to the dual mode. Then, either the MS or the network can initiate a disconnect of the CS connection, which causes the circuit switched call to be released at call control level and subsequently the channel is released.
In FIG. 2, the difference between the “release” and the subsequent “channel release” is as follows. The “RELEASE” message is a GSM Call Control protocol message, which merely releases the circuit-switched call at the Call Control level. Note that this message exchange (RELEASE, RELEASE COMPLETE) does not occur with all dedicated connections, such as Short Message Service (SMS) or MM Location Update. Regarding the ‘CHANNEL RELEASE’ message in FIG. 2, that is a GSM Radio Resource protocol message which indicates that the Radio Resource (i.e. channel) is being released, after which the MS returns to (packet) idle mode according to FIG. 2. Thus, the two ‘release’ messages belong to different protocol entities. In FIG. 2, the MS initiates the disconnect of the CS connection, and the MS then transfers to the packet idle state 214 from which the MS must perform a complete acquisition of system information in order to get back into a packet switched session 216.
If the network supports a Packet Broadcast Control Channel (PBCCH), then the MS will not perform packet access or enter the packet transfer mode 216 until it has acquired the PACKET SYSTEM INFORMATION TYPE 1 (PSI1) message, and acquired a consistent set of PSI2 messages, and also made at least one attempt to receive the complete set of PSI messages on PBCCH. See 3GPP TS 44.060, “Radio Link Control/Medium Access Control (RLC/MAC) protocol” and 3GPP TS 45.008, “Radio Subsystem Link Control.” If the network supports the PACKET PSI STATUS message, the mobile station may perform packet access, and enter packet transfer mode 216, as soon as the PSI1 message and a consistent set of PSI2 messages have been received.
On the other hand, if the PBCCH is not present in the network, then the MS must perform a complete acquisition of Broadcast Control Channel (BCCH) messages, in which case the mobile station will not perform packet access or enter the packet transfer mode 216 until it has acquired the SYSTEM INFORMATION TYPE 3 (SI3), SI13 and, if present, SI1 messages, and additionally has made at least one attempt to receive other SI messages that may be scheduled within one TC cycle on BCCH. TC is a formed mathematical expression of a GSM “multiframe modulo.” The TC value is cyclic and runs from values 0 to 7 (i.e. the TC can have values TC=0, TC=1, TC=2, . . . TC7). One GSM multiframe (on BCCH/CCCH) consists of 51 TDMA frames, adding up to 51 times 60/13 ms which equals approximately 235 ms. Therefore, 8 multiframes (i.e. TC0 . . . TC7) adds up to approximately 1.8 seconds. The reason for quoting the TC value in the context of the present invention is to establish the significant delay experienced from the SYSTEM INFORMATION RECEPTION on the BCCH, in case the CS connection needs to be released before packet access is again possible for the MS (as is specified now according to the prior art).
If the network supports the PACKET SI STATUS message, the MS may perform packet access, and enter packet transfer mode, as soon as the SI3, SI13 and, if present, SI1 messages have been received.
The main problem with these prior art techniques is that the MS is not allowed to immediately enter the packet transfer mode 216 until it has performed various steps. Thus, the mobile station will be forced to idle its packet switching capabilities, while it sets up the packet switching session 216. The related U.S. application “Enhancement of Dual Transfer Mode When Circuit Switched Resources Are Released” addresses this problem, and the present invention also addresses this problem (this related U.S. application is similar to 3GPP Tdoc G2-040288). The present invention covers some issues that this related U.S. application did not address: first, the indication of system information delivery on the packet associated control channel (PACCH), and second, system information handling in the MS, based upon validity time. A possible problem with the solution of the application “Enhancement of Dual Transfer Mode When Circuit Switched Resources Are Released” is that it is possible for a gap to occur in the PS session after the CS connection release, and the present invention solves this possible problem. For further background regarding system information messages, see 3GPP TS Tdoc GP-041144, “Introduction of non-segmented provision of serving cell SYSTEM INFORMATION messages on PACCH.”